The use of redispersible polymer powders as cement modifiers is known. However, in order for the polymer powder to be effective, the following critical requirements have to be met: it should be capable of flowing freely, it should have a long storage life and it should be redispersible in water. Additionally, the polymer powders should be suitable in shearbond adhesion applications and should have superior properties when the powder is used to modify thin sections of cement. These properties are fulfilled only partially by powders which are prepared utilizing prior art blends, mixtures and core-shell polymer processes.
Mixtures and blends of soluble resins with insoluble emulsion polymers are known in the art and are generally used both alone and in various formulations as coatings and impregnates for cementitious applications. The soluble resin is generally prepared by solution polymerization, such as described in U.S. Pat. No. 3,037,942.
It is also known that core-shell emulsion polymers made significant improvements over prior art mixtures and blends such as described in U.S. Pat. No. 4,916,171. By polymerizing one component in the presence of the other to form a core-shell emulsion polymer, improvements in stability, rheology and water resistance were realized. It is still further known that the addition of an emulsion acrylic polymer as a powder dispersion in cementitious mixtures improves properties such as adhesion and strength of the cement matrix. For example, latex modified Portland cement is a premixed material wherein a polymer is added to the fresh concrete mixture in a liquid, powder or dispersed phase and cured. However, while the addition of core-shell emulsion polymers have improved some properties of premixed cement, other deficiencies still remain such as, for example, stability and redispersability of the acrylic emulsion.
What is required is a novel generation of core-shell polymer powders that are easily redispersible, stable, and that do not significantly retard mortar curing, while maintaining other properties such as shear bond adhesion, strength and trowelability.